SANTA CRUZ, Calif., July 30 (UPI) -- Neither the Earth nor its moon are a perfect sphere. Scientists have know this. But while the Earth's slightly elliptical shape has been explained by its rotation and the gravitational pull of the moon, these factors didn't entirely explain the Moon's lemon-like form. And astronomers remained a bit puzzled -- until now.

Researchers at the University of California, Santa Cruz (UCSC) say that tidal forces early in the moon's formational history account for its odd appearance. Today, the moon rotates too slow and is too far from Earth to explain its flattened shape. But in its earliest days, while still predominantly liquid, the moon was closer and spinning faster. The sculpting effects of these two factors were frozen into place as the moon solidified and matured.

When the moon first cooled, only its crust hardened -- a molten core remained, an ocean of liquid rock. The pull of Earth's gravity manipulated the moon's soft insides.

"If you imagine spinning a water balloon, it will start to flatten at the poles and bulge at the equator," explained Ian Garrick-Bethell, assistant professor of Earth and planetary sciences at UC Santa Cruz. "On top of that you have tides due to the gravitational pull of the Earth, and that creates sort of a lemon shape with the long axis of the lemon pointing at the Earth."

"So there's a variety of interesting things that could happen, at that time when the Moon was really hot, that could change its shape," he added.

Garrick-Bethell is the lead author of the study explaining the moon's odd shape. The study -- enabled by new ways of measuring the moons surface that account for missing pieces (blown off by meteor impacts) -- was published this week in the journal Nature.

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